Field of the Invention
The present invention relates to a vertical resonator laser diode (VCSEL vertical-cavity surface-emitting laser), in which an active layer sequence for generating laser radiation is disposed between a first Bragg reflector layer sequence and a second Bragg reflector layer sequence, each of which has a plurality of mirror pairs. The two Bragg reflector layer sequences form a laser resonator and are disposed, together with the active layer sequence, between a first and a second electrical contact layer. One of the two Bragg reflector layer sequences is partially transmissive or semitransparent for the laser radiation generated in the active layer sequence. On at least one of the two Bragg reflector layer sequences, a current constriction device, called "current aperture" below, is provided which serves to concentrate the operating current lowing through the active layer sequence during operation of the laser diode and thus to limit the cross section of the pumped active region of the active layer sequence. The current aperture has at least one aperture layer disposed between one of the electrical contact layers and the active layer sequence. Furthermore, the invention relates to a method for producing such a vertical resonator laser diode.
Such vertical resonator laser diodes with a current aperture 119 are disclosed in U.S. Pat. No. 5,493,577. One example of such a vertical resonator laser diode is illustrated schematically in FIG. 3. In this example, the current constriction (the current flow is indicated by the arrows 130) to the desired pumped region 121 of the active layer sequence 103 is effected through the use of two thin AlAs or AlGaAs aperture layers 105, which are oxidized except for the respective current passage opening 116.
The two aperture layers 105 are disposed on mutually opposite sides of the active layer sequence 103, in each case between the active layer sequence 103 and the Bragg reflector layer sequence 102, 104. The Bragg reflector layer sequences 102, 104 each essentially include a plurality of mirror pairs, each of which has two AlGaAs layers having different band gaps. The Bragg reflector layer sequences 102 is disposed on a substrate 120. The bottom of the substrate 120 is provided with a contact layer 114.
In the oxidized annular regions 122, which define the size and form or shape of the current passage openings 116, the AlAs or AlGaAs aperture layers 105 have a very high electrical resistance. As a result, the pump current 130, which is supplied through a contact layer 115, essentially flows only in the region of the current passage openings and thus in the desired pumped region 121 through the active layer sequence 103.
With regard to a simple production of such vertical resonator laser diodes, the AlAs or AlGaAs aperture layers 105 have a higher Al content than the AlGaAs layers of the mirror pairs of the Bragg reflector layer sequences 102, 104.
This is because the lateral oxidation rate of Al.sub.x Ga.sub.1-x As layers during a heat treatment in a humid atmosphere depends on the Al content (the higher the Al content, the higher the oxidation rate). For this reason, the AlAs or AlGaAs aperture layers 105 as described in the U.S. Pat. No. 5,493,577, can be produced in a simple manner after the production of the laser diode layer structure with the active layer sequence 103, the Bragg reflector layer sequences 102, 104 and the AlAs or AlGaAs layers for the aperture layers 105 by an oxidation of the entire layer structure in a humid atmosphere.
Furthermore, the thin AlAs or AlGaAs aperture layers 105 cause only minor optical losses in the laser resonator. Consequently, it is possible to produce vertical resonator laser diodes having a very high efficiency and small threshold currents.
A disadvantage of the above-described structure of vertical resonator laser diodes, however, is that a severe current crowding occurs at the edges of the current passage openings 116 during operation. This current crowding causes a severe local heating of the Bragg reflector layer sequences 102, 104 and of the active layer sequence 103 in the region of these edges, which accelerates the ageing of the laser diode component. Furthermore, there is also the risk of the locally very high current density leading to the generation and migration of crystal defects, which reduces the lifetime and reliability of the vertical resonator laser diodes.